Which one of the following is not a method of concentration of metals?

$NaCN$ is sometimes added in the froth flotation process as a depressant when $ZnS$ and $PbS$ minerals are expected because

Bauxite containing impurities of iron oxide is purified by

Select the correct option regarding the method of separating two sulfide ores by the froth flotation process.
$(1)$ The ratio of water and oil is chosen appropriately.
$(2)$ Some depressants are used for this process.
$(3)$ $HCN$ is a depressant for a mixture of $ZnS$ and $PbS$.
$(4)$ By using a suitable depressant for a mixture of $ZnS$ and $PbS$,only $PbS$ comes up with the froth.

$PbS$ can be separated from $ZnS$ by electrostatic separation. Which property is involved in this method?

Which principle is involved in magnetic separation?